These investigations are aimed at determining the effects of temperature, humidity and light intensity on the decay process of the organophosphate pesticides on the foliage of various crops. In particular, interest is focused on the formation and degradation of the oxygen analogs of the parent organophosphates, compounds of particular concern as they relate to occupational exposure to farmworkers to weathered residues. In order to control the environmental variables, these studies are being carried out in a large environmental chamber that can accommodate up to 10 dwarf Valencia orange trees. The model system presently under investigation is ethyl parathion applied to these Valencia orange trees. Much of the initial emphasis has been placed on estimating the variance associated with the residue measurement techniques in order to establish the degree of variability in the decay process that will be detectable in samples taken from the environmental chamber.